Stable and high solids aqueous dispersions of blocked polyisocyanates

ABSTRACT

A stable and high-solids aqueous dispersion of blocked isocyanates, containing auxiliary solvent, is prepared by (1) preparing a solution of a polyisocyanate mixture of from 20-70% by weight of a blocked, hydrophilically modified polyisocyanate (A) and from 30-80% by weight of a blocked, hydrophobic polyisocyanate (B) in an auxiliary solvent (G) and (2) adding water to this solution, with intensive mixing, in an amount which is such as to give a two-phase system comprising a disperse polyisocyanate phase and a continuous, aqueous phase containing auxiliary solvent. In a preferred embodiment of the invention dispersions are prepared which are virtually free from auxiliary solvent, by first adding a portion of water to the solution (1), substantially removing the auxiliary solvent with intensive mixing and then adding a second portion of water with intensive mixing to the mixture comprising the polyisocyanates (A) and (B) and the first portion of water.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to stable and high-solids aqueousdispersions of blocked polyisocyanates, either containing auxiliarysolvent or being virtually free from auxiliary solvent, to a process forpreparing such dispersions and the use of such dispersions ascross-linkers for film-forming resins.

[0003] 2. Description of the Background

[0004] Blocked (or capped) polyisocyanates are known and widely used ascross-linking agents for film-forming resins in solvent-borne coatingsystems. For reasons of environmental protection and, of course, of costas well, numerous technical developments of the recent past have beendirected to reducing the amount of solvent or, as is particularlydesirable, at providing solvent-free or virtually solvent-free systems.

[0005] Aqueous dispersions of blocked polyisocyanates are suitable foruse in single-component waterborne (or water-thinnable) coatingmaterials, i.e. in aqueous, solvent-free or virtually solvent-freecoating systems, which, in addition to the blocked polyisocyanates,which function as cross-linking agents, comprise a water-dissolved or-dispersed film-forming resin having functional groups that are able toreact with isocyanate groups.

[0006] Since blocked isocyanates are, in the majority of cases, solidsor highly viscous substances, they can be dispersed in water only withthe addition of solvents or, without solvent, only above their meltingpoint. The disadvantages of the two processes are that, in the firstcase, the solvent used for dilution must be removed again afterdispersion, at considerable expense, while in the case of meltemulsification, some of the blocking agent is cleaved off early at thehigh temperatures required, and the isocyanate groups that have beenreleased react with water in the course of dispersion and are no longeravailable for cross-linking with the functional groups of thefilm-forming resin.

[0007] The aqueous dispersions of the blocked polyisocyanates (and, ofcourse, those of the film-forming resins as well) should, wherepossible, be of high solids content. The principal advantage ofhigh-solids content dispersions is that, with only one application, itis possible to produce thick coats having excellent properties(fullness, resistance, hiding power). Relative to multicoatapplications, this means not only a saving in time and (because of themultiple stoving procedures) in energy, but also in optimizing quality,since each additional coat increases the risk of production errors. Inaddition, high-solids dispersions also lessen the costs oftransportation and storage. The dispersions should, moreover, be stable,i.e. storable for long periods without sedimenting or coagulating to anymarked extent. Should coagulation or sedimentation nevertheless occur,the deposits should be readily and durably redispersible.

[0008] Known processes for preparing stable aqueous dispersions ofblocked polyisocyanates make use of external emulsifiers and protectivecolloids (JP 4 216 815, JP 4 216 816, JP 4 216 817 and WO 94/22935). Adisadvantage of these processes is the permanent hydrophilicity of theemulsifiers and of the protective colloids, which is retained in thecoatings which result on stoving by reaction of the polyisocyanates withthe isocyanate-reactive groups of the film-forming resins. Consequently,these coatings have a tendency, under the effect of water, to becomecloudy, to soften and/or to swell up, ruling out the use of suchdispersions at least for coating materials that are to be employedoutdoors.

[0009] The process that DE 24 56 469 describes for preparing aqueousdispersions starts with partially blocked polyisocyanates, which arereacted with a hydrophilicizing agent, which contains an NCO-reactivegroup and also a hydrophilic or potentially hydrophilic group, forexample, a sulfo acid or carboxyl group. Both types of groups can beconverted by neutralization into hydrophilic ionic groups, for examplesulfonate or carboxylate groups. The hydrophilicizing agents used in theexamples are the sodium salts of N-methylaminoethanesulfonic acid. Thereaction product is then dispersed in water. A similar process isdescribed in EP 0 012 348, where the blocked hydrophilicizedpolyisocyanates are not only self-dispersible, but also promote thedispersion of hydrophobic film-forming resins. In the case of thisprocess, however, which is not used industrially, the linking of thecompound having a (potentially) hydrophilic group takes place, becauseof the partial blocking beforehand, at low NCO contents, which firstlyrequires an uneconomically long reaction time and secondly has theconsequence either that not all of the free NCO groups arehydrophilically linked or, in the case of excess hydrophilicizing agent,that unreacted hydrophilicizing agent remains in the polylsocyanatedispersion.

[0010] DE 27 08 611 describes a process for preparing polyurethaneprepolymers, in which polyisocyanates are first of all reacted with anexcess of hydroxycarboxylic acids such as dimethylolpropionic acid, andthen the free isocyanate groups that remain are blocked. Thesepolyurethane prepolymers must have high acid numbers of >30 mg of KOH/g,since at lower acid numbers, stable dispersions or solutions can beprepared only with additional organic solvents. A disadvantage of thisprocess is that a very considerable proportion of the originalisocyanate groups is lost for cross-linking with the film-forming resinas a result of the reaction with the hydroxycarboxylic acidhydrophilicizing reagent. A further disadvantage of this process is thehigh viscosity of dispersions with a solids content >35 % by weight,which makes processing more difficult if not virtually impossible.

[0011] EP 0 312 836 describes a process in which isocyanate groups arecapped by reaction with a cyanourea salt. The hydrophilic groups therebycreated participate in the cross-linking reaction in the course ofstoving, and thus do not impair the density of cross-linking. Adisadvantage, however, is that amines are given off during stoving,which only with difficulty leave the paint film, and, in the case oflight-colored coatings, cause disruptive yellowing of the coatings.

[0012] EP 0 133 223 describes blocked polyisocyanates which arehydrophilicized by means of cyclic malonates. Curing in these systemstakes place likewise through the cyclic malonate groups, with the resultthat the hydrophilic modification again does not cause any reduction inthe cross-linking density. Detrimental to this process, however, are theconsiderable amounts of water-soluble organic solvent required.

[0013] EP 0 524 511 discloses a process in which nonionically modifiedpolyisocyanates having free isocyanate groups are first of all dispersedin water and then the isocyanate groups are blocked. In the course ofthis reaction, however, some of the isocyanate groups react with waterand are lost for the cross-linking reaction. Moreover, it is difficultto establish reproducibly the particular stoichiometric proportions thatare desired.

[0014] Furthermore, EP 0 537 578 discloses a process in which blockedpolyisocyanates, for textile finishing (crease-resistant finishing), arehydrophilicized with, inter alia, polyalkylene oxide units incorporatedtherein and are dispersed. EP 0 424 697 and DE 28 14 815 also describedispersions of blocked, nonionically hydrophilicized polyisocyanates.These dispersions, however, are in turn permanently hydrophilic, so thatcoatings produced with them may turn cloudy, soften and/or swell underthe action of moisture and are, therefore, unsuitable for exteriorapplications.

[0015] EP 0 022 452 describes a one-component polyurethane system whosehardener is a polyurethane prepolymer in which each molecule is linkedwith a hydrophilic modifying reagent. In addition to the loss ofcross-linkable isocyanate groups, a disadvantage here is that a solventmixture of water and alcohol is required and, in particular, thatdispersions with a solids content of >40% by weight have such a highviscosity that they are difficult to process.

[0016] Finally, EP 0 566 953 and EP 0 576 952 describe specificpolylsocyanate mixtures, hydrophilicized by means of hydroxycarboxylicacids, which are suitable for the curing of film-forming resins inaqueous systems. A need, therefore, continues to exist for an aqueousdispersed polyurethane system of improved stability and high solidscontent.

SUMMARY OF THE INVENTION

[0017] Accordingly, one object of the present invention is to provide anaqueous dispersion of a blocked polyisocyanate which is stable and has ahigh solids content.

[0018] Briefly, this object and other objects of the present inventionas hereinafter will become more readily apparent can be attained by aprocess for preparing stable and high-solids aqueous dispersions ofblocked polyisocyanates, containing auxiliary solvent, comprising:

[0019] (1) preparing a solution of a polyisocyanate mixture of from20-70% by weight of a blocked, hydrophilically modified polyisocyanate(A) and from 30-80% by weight of a blocked, hydrophobic polyisocyanate(B) in an auxiliary solvent (G), and

[0020] (2) adding to this solution, with intensive mixing, an amount ofwater which is sufficient to give a two-phase system comprising adispersed polyisocyanate phase and a continuous, aqueous phasecontaining auxiliary solvent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In an advantageous embodiment of this process, correspondingdispersions which are, however, virtually free from auxiliary solvent(G) are prepared by

[0022] (3) adding a first portion of the total amount of water used tothe solution of stage (1),

[0023] (4) substantially removing the auxiliary solvent (G) from theaqueous mixture that is formed, containing auxiliary solvent, withintensive mixing, and

[0024] (5) adding a second portion of water to the remaining mixturecomprising the blocked polyisocyanates (A) and (B) and the first portionof water, with the provisos that (i) the first portion of water is of asize such that the polyisocyanates (A) and (B) form the continuous phasein the mixture which remains after the substantial removal of theauxiliary solvent (G), and (ii) the addition of the second portion ofwater takes place with intensive mixing, at least up until the point ofphase inversion.

[0025] Another aspect of the invention are stable and high-solidsaqueous dispersions of blocked polyisocyanates, both containingauxiliary solvent and, in particular, dispersions which are virtuallyfree of auxiliary solvent, which dispersions comprise, as a dispersephase, a polyisocyanate mixture of from 20-70% by weight of a blocked,hydrophilically modified polyisocyanate (A) and from 30-80% by weight ofa blocked, hydrophobic polyisocyanate (B), which dispersions areprepared by the process of the invention. The stable and high-solidsdispersions of the blocked polylsocyanates (A) and (B) function ascross-linkers for film-forming resins.

[0026] Properties and Advantages of the Dispersions of the Invention

[0027] The process of the invention provides, surprisingly, dispersionswhich have an optimum combination of the following advantageousproperties:

[0028] (i) The solids content is high. In the case of the systemscontaining auxiliary solvent, the solids content is generally at least35 and up to 80% by weight, preferably from 35-60% by weight. In thecase of the systems which are virtually free from auxiliary solvent, thesolids content is generally in the range from at least 35% by weight upto 60% by weight, advantageously from 40-50% by weight.

[0029] (ii) The viscosities of the high-solids dispersions at 25° C.,despite the high solids content, are generally <1000 mpa·s, with theresult that they can be processed without problems.

[0030] (iii)The latent isocyanate content, i.e. the amount by weight ofisocyanate groups, calculated in unblocked form, in the blockedpolyisocyanate mixture comprising (A) and (B), is sufficiently highdespite the hydrophilicization. It is in general >8% by weight and canbe up to 15% by weight. The mean latent NCO functionality, depending onthe poiyisocyanate employed, is generally in the range from 2.5-4.5 andto can be up to 6.0 or more, so that the particular requirement of highcross-linking density in the stoved coatings is ensured. The high latentisocyanate content and the high cross-linking density provideoutstanding mechanical properties such as hardness and chemicalresistance of product cross-linked coatings. To this extent, thecoatings prepared with the dispersions of the invention are at leastequal to the coatings prepared from the prior art.

[0031] (iv) When low-boiling auxiliary solvents (G) are used, they canbe substantially removed by distillation. In the case of the preferredembodiment of the invention, the solvent is removed following theaddition of a first portion of the total amount of water used, as aresult of which the content of residual auxiliary solvent (G) in thefinished dispersion, as formed by adding the second portion of water,can be reduced without difficulty to <2% by weight, in particular <0.5%by weight. The dispersions are, therefore, virtually free from auxiliarysolvent. Contents of residual auxiliary solvent (G) of <0.5% by weightare advantageous since the dispersions do not give rise to anyunpleasant odor in the course of the processing.

[0032] (v) The dispersions of the invention are storable for longperiods both at room temperature and at slightly elevated temperature upto 60° C. For example, dispersions virtually free from auxiliary solventwith solids contents of from 45-50% by weight produce <1% by weight ofsolid sediment under optimum conditions at room temperature within 6months or at 60° C. within 4-8 weeks. The slight sedimentation is ingeneral, moreover, readily and durably redispersible. This good toexcellent long-term stability is achieved with a minimum ofhydrophilicity, namely by hydrophilicizing only part of thepolyisocyanate, without the addition of an external dispersant and/orprotective colloid and without the incorporation of hydrophilicizingpolyoxyalkylene units and also, at least in the case of the preferreddispersions, without the addition of solvent.

[0033] (vi) The minimal hydrophilicity has the desired consequence.moreover, that the cured coatings are not very sensitive to moisture andare, therefore, also well-suited to exterior applications.

[0034] An essential feature of the invention is that the blockedpolylsocyanate is a mixture of from 20-70% by weight of a blocked,hydrophilically modified polyisocyanate (A) and from 30-80% by weight ofa blocked, hydrophobic polyisocyanate (B). The hydrophilically modifiedpolyisocyanates (A) apparently facilitate the dispersion of thehydrophobic polyisocyanates (B) to give dispersions having optimumproperties in terms of performance, such as viscosity, particle size andhardness of the subsequent coating films, and provide long-termstabilization of the dispersions. Advantageously, the proportion of theblocked, hydrophilically modified polyisocyanate (A) is from 35-60% byweight and, in particular, from 40-50% by weight. The proportion of theblocked, hydrophobic polyisocyanate (B), accordingly, is advantageouslyfrom 40-65% by weight and, in particular, from 50-60% by weight.

[0035] The Hydrophilically Modified Polyisocyanates (A) and theHydrophobic Polvisocyanates (B)

[0036] Both the blocked, hydrophilically modified polyisocyanates (A)and the blocked, hydrophobic polyisocyanates (B) are derived from thecustomary hydrophobic polyisocyanates (C) employed in paint chemistry.Instead of using only one blocked, hydrophilically modifiedpolyisocyanate (A) and/or only one blocked, hydrophobic polyisocyanate(B), it is not infrequently possible to employ, with advantage,polyisocyanates (A) and/or (B) which are based on mixtures of differenthydrophobic polyisocyanates (C). The hydrophobic polyisocyanates (C)become hydrophilically modified polyisocyanates (D) by being reactedwith a hydrophilicizing agent (E) and, if desired, with a neutralizingagent (H), as explained later on. By reaction with a blocking agent (F)the hydrophilically modified polyisocyanate (D) becomes the blocked,hydrophilically modified polyisocyanate (A) and the hydrophobicpolyisocyanate (C) becomes the blocked, hydrophobic polyisocyanate (B).

[0037] In principle, suitable polyisocyanates for the invention includeall aliphatic, cycloaliphatic, araliphatic, aromatic or heterocyclicpolyisocyanates, as are described, for example, by W. Siefken in LiebigsAnnalen der Chemie, 562, pages 75-136. A detailed exposition of thepolyisocyanates that can be employed is given in EP 0 537 578, page 3,lines 10-45. Preferred polyisocyanates, because of the high lightstability and weathering resistance of the paint films producedtherefrom, include aliphatic and/or cycloaliphatic polyisocyanateshaving a mean molecular weight of up to about 1000 g/mol, advantageouslyof about 800 g/mol, and with a mean isocyanate functionality of from2-4. Examples of these include simple diisocyanates such as1,6-diisocyanatohexane (HDI), bis(4-isocyanatocyclohexyl)methane (HMDI),1,5diisocyanato-2-methylpentane (MPDI),1,6-diisocyanato2,4,4-trimethylhexane and/or1,6-diisocyanato-2,2,4-trimethylhexane (TMDI), and also, in particular,3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophoronediisocyanate, IPDI). They also include dimers of uretdione structureprepared by catalytic reaction of these simple diisocyanates,principally of IPDI, HDI and/or HMDI. Another preferred class ofpolyisocyanates are the compounds having more than two isocyanate groupsper molecule, that are prepared by allophanatization, trimerization,biuretization or urethanization of the simple diisocyanates, for examplethe reaction products of these simple diisocyanates such as IPDI, HDIand/or HMDI, with polyhydric alcohols (for example glycerol,trimethylolpropane, pentaerythritol) or with polyfunctional polyamines,or the triisocyanurates prepared by trimerizing the simple diisocyanatessuch as IPDI, HDI and HMDI. Examples of the (less preferred) aromaticpolyisocyanates that may be mentioned include 2,4-and/or2,6-diisocyanatotoluene and also 4,4′- and/or2,4′-diisocyanatodiphenylmethane.

[0038] The Hydrophilicizing Agent (E)

[0039] As mentioned, the hydrophobic polyisocyanates (C) can beconverted by reaction with a hydrophilicizing agent (E) and, if desired,with a neutralizing agent (H), as set out below, into hydrophilicallymodified polyisocyanates (D). Suitable hydrophilicizing agents (E)include at least one NCO-reactive group having a hydrogen that is activein accordance with the Zerewitinoff test and at least one hydrophilic orpotentially hydrophilic group. Suitable NCO reactive groups, inparticular include hydroxyl groups and primary and secondary aminogroups. Hydrophilic groups include, inter alia, again hydroxyl groups,which are present in the hydrophilicizing agent or may come about as aresult of groups present in the hydrophilicizing agent. Examples ofpreferred hydrophilic groups are the sulfo acid group and the phosphonicacid group. These are comparatively strongly dissociated relative to thecarboxyl group and are, therefore, regarded as hydrophilic. They can beconverted by neutralization into the ionic hydrophilic sulfonate andphosphonate groups, respectively, thereby raising the pH of thedispersion into the neutral or basic range, which is generallyadvisable. The carboxyl group is an example of a potentially hydrophilicgroup, since it dissociates but weakly and is, therefore, hardlyhydrophilic, but is converted by neutralization almost entirely into thestrongly dissociated, ionic hydrophilic carboxylate group. Anotherexample of a potentially hydrophilic group is the tertiary amino group,which is converted by reaction (or neutralization) with an acid into theionic hydrophilic, quaternary ammonium group. The hydrophilicizing agent(E) must, of course, only be employed in an amount such that sufficientisocyanate groups for the cross-linking reaction with the film-formingresin are retained.

[0040] A detailed description of the hydrophilicizing agents ispresented in EP 0 537 578. Examples of suitable hydrophilicizing agents(E) include hydroxycarboxylic, hydroxysulfonic, hydroxyphosphonic,aminocarboxylic and aminosulfonic acids and tertiary aminoalkanols.Specific examples include glycolic acid, betaaminopropionic acid,hydroxyethanesulfonic acid, hydroxyethanephosphonic acid andN,N-dimethylaminoethanol. A particularly suitable hydrophilicizing agent(E) is dimethylolpropionic acid (DMPA), each of whose hydroxyl groupsreacts with one isocyanate group and which is, therefore, able to linktwo molecules of polyisooyanate with an increase in the molecular weightand, if appropriate, in the NCO functionality as well. Probably as aresult of steric hindrance, the carboxyl group is virtually inert withrespect to the isocyanate groups under the reaction conditions.

[0041] Instead of reacting a hydrophobic polyisocyanate (C) to form ahydrophilically modified polyisocyanate (D) and mixing the latter withfurther hydrophobic polyisocyanate (C) for conjoint blocking, themixture of the polyisocyanates (A) and (B) can also be produced by onlypartially hydrophilicizing an amount of hydrophobic polyisocyanate (C)corresponding to this mixture by reacting it with an amount ofhydrophilicizing agent (E) which is such that the proportions ofresultant, hydrophilically modified polyisocyanate (D) and of remaininghydrophobic polyisocyanate (C) correspond to the desired ratio ofblocked hydrophilically modified polyisocyanate (A) to blockedhydrophobic polyisocyanate (B) for the finished dispersion, byconverting any potentially hydrophilic groups by neutralization intoionic hydrophilic groups and by blocking the unreacted isocyanate groupsof the hydrophilicizing mixture.

[0042] The Blocking of the Hydrophilically Modified polvisocyanate (A)and of the Hydrophobic Polyisocyanate (B)

[0043] The isocyanate groups of the hydrophilically modifiedpolyisocyanate (D) and of the hydrophobic polyisocyanate (C) are blockedin conventional manner, in each case individually or, preferably,together, by reaction with appropriate blocking agents (F). Blocking isdescribed, for example, in Progress in Organic Coatings 3 (1975), pages73-99 and in Progress in Organic Coatings 9 (1981), pages 3-28. Use ismade of the known blocking agents (F), which at from 20-120° C. enterwith isocyanate groups into an addition reaction which is reversible athigher temperatures, so that the isocyanate groups that are then freedagain are able to react with reactive groups of a film-forming resin.Examples of appropriate blocking agents (F) include secondary andtertiary alcohols, phenols, C-H-acidic compounds such as malonic acidderivatives, lactams, e.g., ε-caprolactam and oximes. Preferred blockingagents include oximes such as formaldoxime, acetaldoxime, cyclohexanoneoxime, acetophenone oxime, benzophenone oxime and, in particular, methylethyl ketoxime. Other blocking agents which can be used are3,5-dimethylpyrazole and 1,2,4-triazole.

[0044] The Neutralizing Agent (H)

[0045] If a potentially hydrophilic group, for example a carboxyl groupor a tertiary amino group, is introduced into the polyisocyanate by thehydrophilicizing agent (E), it can be converted by the neutralizingagent (H) into an ionic hydrophilic group. The neutralizing agent is abase if the potentially hydrophilic group is an acidic group, forexample the carboxyl group, and is an acid in the case of basicpotentially hydrophilic groups, for example a tertiary amino group. Thebases can be inorganic bases such as ammonia or hydrazine, or organicbases. Preference is given to ammonia and to primary, secondary ortertiary amines, such as ethylamine, n-propylamine, dimethylamine,di-n-butylamine, cyclohexylamine, benzylamine, morpholine, piperidineand triethanolamine. Particular preference, because of their inertbehavior relative to the blocked NCO functions, is given to tertiaryamines such as N,N-dimethylethanolamine, N,N-diethylaminoethanol,triethylamine, tri-n-propylamine and tri-n-butylamine. Suitable acidspreferably include carboxylic acids such as formic acid, acetic acid andbenzoic acid.

[0046] The Auxiliary Solvent (G)

[0047] Preferred auxiliary solvents (G) include low-boiling inertsolvents, which do not exhibit a miscibility gap with water, at leastover wide ranges. These solvents have a boiling point at atmosphericpressure of <100° C. and can, therefore, be removed easily, if desired,by distillation down to a residual content of <2% by weight and, inparticular, of <0.5% by weight, based on the finished dispersion, andreused. Examples of suitable such solvents include acetone, methyl ethylketone and tetrahydrofuran. Also suitable in principle arehigher-boiling solvents such as n-butylglycol, di-n-butylglycol andN-methylpyrrolidone, which remain in the water-thinnable dispersion.However, they are less preferred because a particular advantage of theinvention is that it is possible to prepare aqueous dispersions whichare high in solids and stable even without environmentally pollutingorganic solvents.

[0048] The Preparation Process

[0049] Hydrophobic polyisocyanate (C) is first of all reacted with thehydrophilicizing agent (E), alone or in the presence of a catalyst. Thehydrophobic polyisocyanate (C) is judiciously employed as a solution inan auxiliary solvent (G) with a solids content of from about 40-80% byweight. Suitable catalysts include organic tin salts such as dibutyltindiacetate or dibutyltin dilaurate. The reaction is started, for example,at room temperature and then the temperature is raised to up to 120° C.in order to complete the reaction. Such relatively high temperatures mayin particular be necessary when blocking is conducted without solvent.When an auxiliary solvent (G) of appropriate boiling point is used aswell, the reaction mixture can be heated at reflux for a time. It ispreferable to operate at a temperature from 40-100° C. At thesetemperatures, the potentially hydrophilic groups of the hydrophilicizingagent (E) are generally inert, or virtually inert, with respect to theisocyanate group.

[0050] The hydrophilicizing agent is advantageously employed in anamount such that on average there is not more than one NCO-reactivefunction in the hydrophilicizing agent, intended for linking, perpolyisocyanate molecule. In the case of dimethylolpropionic acid,therefore, not more than 1 mol of the acid is employed for 2 mol ofpolyisocyanate. It is advantageous to use about 1 equivalent of theNCO-reactive function that is intended for linking per mole ofpolyisocyanate. In each case, a mixture is formed that has statisticaldistribution of the hydrophilicizing groups. This mixture is regarded asa hydrophilically modified polyisocyanate (D) in the context of theinvention, even if it does not include hydrophilically modifiedcomponents.

[0051] To block the isocyanate functions it is also possible to treat amixture of the resultant, hydrophilically modified polyisocyanate (D)and hydrophobic polyisocyanate (C), the latter being in turn, ifdesired, in the form of a from 40-80 percent strength by weight solutionin an auxiliary solvent (G), with the blocking agent (F) in the statedproportion. As mentioned, the mixture to be blocked can also be producedby reacting hydrophobic polyisocyanate (C) with a corresponding excessof hydrophilicizing agent (E) and, if desired, with neutralizing agent(H). Alternatively, but less practically, the hydrophilically modifiedpolyisocyanate (D) and the hydrophobic polyisocyanate (C) can each beblocked separately-and the blocked products mixed. To make the mixingprocedures easier it is possible in any case to add a quantity (orpossibly a further quantity) of auxiliary solvent (G). Blocking takesplace, depending on the blocking agent, at room temperature orjudiciously at an elevated temperature of from 40-100° C. The blockingreaction leads to a temperature-dependent equilibrium. By means ofguideline experiments it is possible without difficulty to determine theoptimum temperature for a given polyisocyanate mixture (C) and (D) and agiven blocking agent (F). The amount of blocking agent (F) depends onthe number of isocyanate functions to be blocked. It is judicious toemploy stoichiometric amounts, or a slight deficit, of blocking agent(F) in order to ensure complete reaction of the blocking agent, so thatneither the product nor the auxiliary solvent (G) to be recycled iscontaminated with blocking agent (F).

[0052] In principle it is also possible first of all to subjecthydrophobic polyisocyanate (C) to partial blocking and then to react theremaining isocyanate groups with the hydrophilicizing agent (E), toconvert any potentially hydrophilic groups into ionic hydrophilicgroups, by neutralization, and to mix the resulting, blocked,hydrophilically modified polyisocyanate (A) with blocked, hydrophobicpolyisocyanate (B). However, this is not advantageous, since thehydrophilicization reaction requires uneconomically long times becauseof the relatively low concentration of isocyanate groups.

[0053] If the hydrophilicizing agent (E) has initially introduced onlypotentially hydrophilic groups, the solution of the blockedpolyisocyanates (A) and (B), as it is or following the addition offurther auxiliary solvent (G), is neutralized in order to convert thepotentially hydrophilic groups into ionic hydrophilic groups. Asmentioned, it is generally advisable to neutralize more strongly acidicand, therefore, hydrophilic groups as well. In both cases, theneutralizing agent (H) is employed in stoichiometric amounts or else indeficient or excess amounts. In general, from 50-130 mol % of the amountrequired for complete neutralization is employed. By way of the amountof neutralizing agent it is possible to influence the viscosity of thedispersion. The greater the deficit, the less viscous the dispersion. Onthe other hand,-the ionic hydrophilic groups formed as a result ofneutralization promote the dispersion of the blocked polyisocyanates, sothat not less than 50 mol % of neutralizing agent should be used. Theoptimum amount also depends on the proportion of blocked hydrophilicallymodified polyisocyanate (A) to blocked hydrophobic polyisocyanate (B)and for a particular dispersion can be determined by experiments withoutdifficulty.

[0054] To prepare aqueous dispersions containing auxiliary solvent,water, which preferably has been deionized, is added to the neutralizedsolution, gradually or all at once. Alternatively, the neutralizedsolution is introduced, again gradually or all at once, into preferablydeionized water. In both cases provision is made for intensive mixing,with or without the action of shear forces, thereby promoting thedevelopment of fine, stable dispersions. Intensive mixing is broughtabout, for example, with a propeller stirrer or with a dissolver. It isalso possible to combine the operations of neutralization anddispersion, for example by adding the neutralizing agent (H) and thewater simultaneously to the solution of the blocked polyisocyanates (A)and (B), it being possible for all or some of the neutralizing agent (H)to be dissolved in the water. The amount of water is preferably suchthat a dispersion with a solids content of from 35-80% by weight and acontinuous phase containing aqueous auxiliary solvent is formed. Itdepends on the nature and the amount of the auxiliary solvent (G) and onthe nature, proportion and degree of hydrophilicization of thepolyisocyanates (A) and (B) and can be determined without difficulty byexperiments. The amount of the auxiliary solvent in the dispersion isgenerally less than 25% by weight.

[0055] The auxiliary solvent-containing dispersions prepared in this wayhave the advantage over the conventional, water-free systems of a lowersolvent content. In comparison with systems containing aqueous auxiliarysolvent and external emulsifiers or protective colloids, they areadvantageous because the ultimate coated film is markedly more resistantto moisture and the effects of weathering. In relation to systemscontaining aqueous auxiliary solvent and extensively or completelyhydrophilicized polyisocyanates, the present dispersions have the sameadvantage; moreover, they are considerably less viscous, whichfacilitates their processing.

[0056] The dispersions virtually free from auxiliary solvent and havinga solids content of from 35-60% by weight, advantageously from 40-50% byweight, that are prepared in the above-mentioned preferred embodiment ofthe process of the invention are particularly desirable. To obtain them,the overall amount of water is divided into two portions. First of all,a first portion is added to the neutralized solution of the blockedpolyisocyanates (A) and (B). This first portion is such that when theauxiliary solvent (G) is removed there is no phase inversion, i.e. thewater remains in dispersion or in solution in the continuouspolyisocyanate phase. A sharp drop in viscosity is characteristic of thephase inversion. The maximum first portion of water which still does notlead to phase inversion depends essentially on-the desired solidscontent of the dispersion and on the nature of the polyisocyanates,their proportion and their degree of hydrophilicization, and can bedetermined without difficulty by experiments. In general it is not morethan 75% of the overall amount of water. The optimum first portion ofwater is in general from 30-60% of the overall amount. The first portionof water can, like the solution of the polyisocyanates (A) and (B), beat room temperature at moderately elevated temperature such as 30-40°C., or else at a high temperature.

[0057] Following the addition of the first portion of water thetemperature is raised, rapidly or gradually, to from 50-110° C. underatmospheric or reduced pressure, so that auxiliary solvent (G) isremoved by distillation. During this procedure intensive mixing is againprovided, as described above. This promotes the formation of a fine,stable dispersion and also brings about rapid and substantial removal ofthe auxiliary solvent (G). “Substantial removal” means that the contentof auxiliary solvent (G) in the finished dispersion after adding thesecond portion of water is not more than 5% by weight. Then, underreduced pressure, it is possible to remove further auxiliary solvent (G)so that its content in the ready-to-use dispersion is <2% by weight,advantageously <0.5% by weight. However, it is more practical to removethe auxiliary solvent (G) even before adding the second portion ofwater, to such a substantial extent that its content in the finisheddispersion, i.e, simply after the addition of the second portion ofwater, is <2% by weight, advantageously <0.5% by weight. When theauxiliary solvent has been substantially removed, the second portion ofwater is added, gradually, at intervals or continuously. In this caseintensive mixing as described above is provided, at least to the pointof phase inversion, so that the resulting dispersion is sufficientlyfine and stable. The temperature of the remaining water isadvantageously in the range in which the temperature of the dispersionlies, while the auxiliary solvent (G) is removed, preferably at from60-100° C. If superatmospheric pressure is used when adding the secondportion of water, the temperature of the water may exceed 100° C. Thismakes the operation of dispersion easier, especially in the case ofsystems of relatively high viscosity. However, it is necessary in doingthis to take note of the deblocking temperatures. Following the additionof the second portion of water, the dispersion is allowed to cool withstirring.

[0058] Stable and high-solids, aqueous dispersions are obtained that arevirtually free from auxiliary solvent and which on storage at roomtemperature for 6 months or at 60° C. for from 4-8 weeks generallydeposit less than 1% by mass of the solids they contain. This amount ofsolids, and any greater amounts of solids which may occasionally bedeposited under suboptimal conditions, can be redispersed rapidly anddurably by the action of appropriately high shear forces. Compared withaqueous dispersions that are virtually free from auxiliary solvent andhave been prepared using emulsifiers or protective colloids, the presentdispersions have the advantage that the ultimate coated films are moreresistant to moisture and the effects of weathering. The same applies incomparison to corresponding dispersions in which there are relativelyhighly, or completely, hydrophilicized polyisocyanates; the latter,moreover, have a considerably higher viscosity, hindering theirprocessing.

[0059] Using the Dispersions

[0060] The stable and high-solids aqueous dispersions of the invention,containing auxiliary solvent or virtually free from auxiliary solvent,are suitable as cross-linking agents for heatcross-linking,storage-stable one-component coating systems, preferably for producingenvironmentally compatible, water-thinnable polyurethane coating systemsby combination with aqueous film-forming resins, i.e. aqueous solutions,dispersions or emulsions and/or other water-thinnable systems comprisingpolymeric resins, having on average more than 1.5 NCO-reactive groupssuch as hydroxyl or amino groups, in each molecule. For this purpose thedispersions of the invention are combined with aqueous film-formingresins, preferably in amounts such that there is one NCO-reactive groupof the film-forming resin per NCO group of the polyisocyanate. Ifdesired, further cross-linkers known to those of skill in the art suchas melamine resins, and/or known auxiliaries, for example, additives toenhance the leveling, the gloss and/or the adhesion of the coating, aremixed in, and the finished coating material is applied as it is orfollowing dilution with water to adjust the viscosity, to the substratein a usual manner. The coating is preferably first of all dried and thenstoved at from 100-250° C. with elimination of the blocking agent,thereby cross-linking the film-forming resin.

[0061] Having generally described this invention, a furtherunderstanding can be obtained by reference to certain specific exampleswhich are provided herein for purposes of illustration only and are notintended to be limiting unless otherwise specified.

EXAMPLE I

[0062] (1.1) Preparing hydrophilic polyisocyanate

[0063] A 741 g amount of IPDI isocyanurate (VESTANAT®1890 from Huels AG)and 222 g of IPDI (VESTANAT®IPDI from Huels AG) are dissolved withstirring in 500 g of acetone. A 22.0 g amount of a 10% strength solutionof dibutyltin dilaurate in acetone, as catalyst, and 134 g of pulverizeddimethylolpropionic acid are added with stirring and the mixture-isheated to about 60° C. so that it boils at reflux. It is stirred untilall the dimethylolpropionic acid has dissolved and the NCO content ofthe solution (determined in accordance with DIN 53185) has fallen tofrom 7.8-8.0%, which takes from 6-8 hours.

[0064] (1.2) Adding the Hydrophobic Polyisocyanate (C) and Blocking

[0065] A 1,482 g amount of IPDI isocyanurate (see section 1.1) and 3,000g of acetone (for dilution) are added with stirring to the solution ofsection 1.1, which is just no longer boiling. The mixture is stirred atfrom 50-60° C. until a homogeneous solution is formed, this solution iscooled to from 40-55° C., and 783.0 g of methyl ethyl ketoxime are addedwith cooling at a rate such that the temperature remains in the rangefrom 40-50° C. The batch is stirred at this temperature for a further 30minutes and then cooled with stirring to 30° C.

[0066] (1.3) Dispersing the Blocked Polyisocyanate (A) and (B)

[0067] To the solution prepared as in section 1.2, 117 g ofN,N-diethylaminoethanol and then, gradually and with stirring, 1,700 gof deionized water are added. The mixture is heated at from 80-95° C. inan open vessel. When almost all of the acetone has evaporated, another2,700 g of deionized water which has been heated to from 70-85° C. isadded with intensive mixing, by means of a propeller stirrer, almostcontinuously over the course of from 5-10 minutes. The resultingdispersion is then left to cool with stirring to 30° C.

[0068] A white dispersion is obtained having a solids content of 45% byweight (in accordance with DIN 53216 at 105° C.), an acetone content of0.3% by weight (by headspace GC), a pH of 8.9 (in accordance with DIN53785) and a viscosity at room temperature of 60 mPa·s (in accordancewith DIN 53019 with D=200 s⁻¹). On storage for 6 months at roomtemperature or for 4 weeks at 50° C., there is no coagulation orsedimentation. The dispersion can be used without restrictions withinthis period.

EXAMPLE 2

[0069] (2.1) Preparing Hydrophilic Polyisocyanate (A), Adding theHydrophobic Isocyanate (C) and Blocking

[0070] The hydrophilic polyisocyanate is prepared as described insection 1.1, and 741 g of IPDI isocyanurate and 222 g of IPDI (as inExample 1) and, for dilution, 1,368 g of acetone are added with stirringto the solution which is just no longer boiling. Stirring is continuedat from 50-60° C. until a homogeneous solution is formed, which iscooled to from 40-55° C., and 696 g of methyl ethyl ketoxime are addedat a rate such that the temperature remains in the range from 40-50° C.The mixture is stirred at this temperature for a further 30 minutes andthen allowed to cool with stirring to 30° C.

[0071] (2.2) Dispersina the Blocked Polyisocyanates (A) and (B)

[0072] A 80 g amount -of -N,N-dimethylaminoethanol and then 1,484 g ofdeionized water are added to the solution prepared in accordance withsection 2.1. The mixture is heated with intensive mixing to from 75-85°C. and is substantially freed from acetone by reducing the pressure tofrom 800-900 mbar. Then four times 470 g of water preheated to from70-80° C. are added slowly at short intervals, with further intensivemixing, and the resulting dispersion is left to cool with stirring to30° C.

[0073] A white dispersion is obtained having a solids content of 44% byweight (in accordance with DIN 53216 at 105° C.), an acetone content of0.3% by weight (by headspace GC), a pH of 8.8 (in accordance with DIN53785) and a viscosity of 50 mPa·s (in accordance with DIN 53019 withD=200 s⁻¹). On storage for 6 months at room temperature or for 4 weeksat 50° C., there is no coagulation or sedimentation. The dispersion canbe used without restrictions within these periods.

EXAMPLE 3

[0074] (3.1) Preparing hydrophilic polyisocyanate (D)

[0075] A 1,482 g amount of IPDI isocyanurate (VESTANAT®T 1890 from HulsAG) is dissolved with stirring in 1000 g of acetone. 3.2 g of dibutyltindilaurate as catalyst and 134 g of pulverized dimethylolpropionic acidare added with stirring and the mixture is heated to about 60° C. sothat it boils at reflux. It is stirred until all the dimethylolpropionicacid has dissolved and the NCO content of the solution (in accordancewith DIN 53185) has fallen to 6.1-6.3%, which takes from 6-8 hours.

[0076] (3.2) Adding the Hydrophobic Polvisocyanate (C) and Blocking

[0077] A 1,734 g amount of HDI isocyanurate (DESMODUR® N 3300 from BayerAG) and 2,500 g of acetone (for dilution) are added with stirring to thesolution of section 3.1, which is just no longer boiling. The mixture isstirred at from 50-60° C. until a homogeneous solution is formed, thissolution is cooled to from 40-55° C., and 1,131 g of methyl ethylketoxime are added with cooling at a rate such that the temperatureremains in the range from 40-50° C. The batch is stirred at thistemperature for a further 30 minutes and then the solution of theblocked polyisocyanates is cooled to 30° C.

[0078] (3.3) Dispersing the Blocked Polvisocyanates (A) and (B)

[0079] To the solution prepared as in section 3.2, 89 g ofN,N-dimethylaminoethanol and then, gradually and with vigorous stirring,2,000 g of deionized water are added. The mixture is heated to from75-90° C. with intensive mixing and is substantially freed from acetoneby reducing the pressure to from 800-900 mbar. Then, almostcontinuously, a further 4,200 g of deonized water that has been heatedto from 80-90° C. are added with further intensive mixing over thecourse of from 10-20 minutes.

[0080] A white dispersion is obtained having a solids content of 42% byweight (in accordance with DIN 53216 at 105° C.), an acetone content of0.4% by weight (by headspace-GC), a pH of 8.9 (in accordance with DIN53785) and a viscosity of 250 mPa·s (in accordance with DIN 53019 withD=200 s⁻1). On storing the dispersion over a period of 6 months at roomtemperature or of 4 weeks at 50° C., there is no coagulation orsedimentation. The dispersion can be used without restrictions duringthese periods.

[0081] (3.4) Example 4 (Non-inventive Comparison Example)

[0082] The initial procedure is as described under sections 1.1 and 1.2,and 89 g of N,N-dimethylaminoethanol and then 4,112 g of deionized waterthat has been heated to from 30-40° C. are added all at once to thesolution of the blocked polyisocyanates (A) and (B). The mixture issubstantially freed from acetone with intensive mixing at from 50-60° C.under reduced pressure. Even during this procedure, there soon occur inhomogenities and precipitation of solids. Ultimately, complete phaseseparation occurs. The white, paste-like mass that has deposited cannotbe redispersed.

[0083] Obviously numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is,therefore, to be understood that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein.

What is claimed as new and is desired to be secured by Letters Patent ofthe United States is:
 1. A process for preparing stable and high-solidsaqueous dispersions of blocked isocyanates, containing auxiliarysolvent, which comprises: (1) preparing a solution of a polyisocyanatemixture of from 20-70% by weight of a blocked, hydrophilically modifiedpolyisocyanate (A) and from 30-80% by weight of a blocked, hydrophobicpolyisocyanate (B) in an auxiliary solvent (G); and (2) adding to thissolution, with intensive mixing, an amount of water which is such as togive a two-phase system comprising a disperse polyisocyanate phase and acontinuous, aqueous phase containing auxiliary solvent.
 2. The processas claimed in claim 1, wherein the blocked, hydrophilically modifiedpolyisocyanate (A) is a blocked, ionic hydrophilically modifiedpolyisocyanate which forms as a result of complete or partialneutralization of a polyisocyanate having a potentially hydrophilicgroup.
 3. The process as claimed in claim 2, wherein the potentiallyhydrophilic group is a carboxyl group and the neutralizing agent isammonia or an amine.
 4. The process as claimed in claim 1, wherein themixture of the polyisocyanates (A) and (B) is produced by only partiallyhydrophilicizing an amount of hydrophobic polyisocyanate (C)corresponding to this mixture by reacting it with an amount ofhydrophilicizing agent (E) which is such that the proportions ofresultant, hydrophilically modified polyisocyanate (D) and of remaininghydrophobic polyisocyanate (C) correspond to the desired ratio ofblocked hydrophilically modified polyisocyanate (A) to blockedhydrophobic polyisocyanate (B) for the finished dispersion, byconverting any potentially hydrophilic groups by neutralization intoionic hydrophilic groups and by blocking the unreacted isocyanate groupsof the hydrophilicizing mixture.
 5. The process as claimed in claim 1,wherein dispersions, which are virtually free from auxiliary solvent areprepared by: (3) adding a first portion of the total amount of waterused in the preparation of the dispersion to the solution of stage (1)(4). substantially removing the auxiliary solvent (G) from the aqueousmixture that is formed, containing auxiliary solvent, with intensivemixing, and (5) adding a second portion of water to the remainingmixture comprising the blocked polyisocyanates (A) and (B) and the firstportion of water, with the provisos that (i) the first portion of wateradded is of an amount such that the polyisocyanates (A) and (B) form thecontinuous phase in the mixture which remains after the substantialremoval of the auxiliary solvent (G), and (ii) the addition of thesecond portion of water takes place with intensive mixing, at least upuntil the point of phase inversion.
 6. A stable and high-solids aqueousdispersion containing auxiliary solvent, which comprises: a dispersephase of a polyisocyanate mixture of from 20-70% by weight of a blocked,hydrophilically modified polyisocyanate (A) and from 30-80% by weight ofa blocked, hydrophobic polyisocyanate (B) having a solids content offrom 35-80% by weight.
 7. The dispersion as claimed in claim 6, whichhas a content of auxiliary solvent (G) of less than 25% by weight.
 8. Astable and high-solids aqueous dispersion which is virtually free fromauxiliary solvent, which comprises: a disperse phase of a polyisocyanatemixture of from 20-70% by weight of a blocked, hydrophilically modifiedpolyisocyanate (A) and from 30-80% by weight of a blocked, hydrophobicpolyisocyanate (B), having a solids content of from 40-60% by weight anda content of auxiliary solvent (G) of <2% by weight.
 9. The dispersionas claimed in claim 8, which has a content of auxiliary solvent (G) of<0.5% by weight.
 10. The dispersion as claimed in claim 6, wherein theblocked, hydrophilically modified polyisocyanate (A) is a blocked,ionic, hydrophilically modified polyisocyanate which forms by completeor partial neutralization of a polyisocyanate having a potentiallyhydrophilic group.
 11. The dispersion as claimed in claim 10, whereinthe potentially hydrophilic group is a carboxyl group and theneutralizing agent (H) is ammonia or an amine.
 12. The process asclaimed in claim 1, wherein said solvent is acetone, methyl ethylketone, tetrahydrofuran, n-butylglycol, di-n-butylglycol orN-methylpyrrolidone.
 13. The process as claimed in claim 10, wherein thepolyisocyanate parent of each of polyisocyanates (A) and (B) is adiisocyanate selected from the group consisting of 1,6-diisocyanatohexane (HDI), bis(4-isocyanatocyclohexyl)methane (HMDI),1,5-diisocyanato-2-methylpentane (MPDI),1,6-diisocyanato-2,4,4-trimethylhexane and/or1,6-diisocyanato-2,2,4-trimethylhexane (TMDI),3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI),uretdione thereof, polyisocyanates having more than two isocyanategroups per molecule prepared by allophanatization, trimerization,biuretization or urethanization of the diisocyanates with polyhydricalcohols, with polyfunctional polyamines, the triisocyanurates preparedby trimerizing the diisocyanates, or aromatic polyisocyanates.
 14. Theprocess as claimed in claim 1, wherein the hydrophilicizing agent is ahydroxycarboxylic, hydroxysulfonic, hydroxyphosphonic, aminocarboxylicor aminosulfonic acids or a tertiary aminoalkanol.
 15. The process asclaimed in claim 14, glycolic acid, betaaminopropionic acid,hydroxyethanesulfonic acid, hydroxyethanephosphonic acid,N,N-dimethylaminoethanol or dimethylolpropionic acid (DMPA).
 16. Theprocess as claimed in claim 5, wherein the amount of said first portionof water is not more than 70 wt % of the total of the amount of wateradded to form the product dispersion.
 17. The process as claimed inclaim 5, wherein at the addition of said first portion of water thetemperature is increased to 50-110° C. under atmospheric or reducedpressure under intensive mixing which results in the removal of water toa concentration of <2% by weight
 18. The process as claimed in claim 6,wherein solvent is removed to a concentration of <0.5% by weight.
 19. Amethod of preparing a film-forming resin, comprising: combining thedispersion of claim 6, as a cross-linking agent, with an aqueousfilm-forming resin in which the resin contains an average of >1.5NCO-reactive groups in each molecule.
 20. The method of claim 19,wherein said film-forming resin is a melamine resin.